"Jay" <eatthisandide@127.0.0.1> wrote in message news:_ZUPb.15984$xu6.9047@fe02.usenetserver.com...> In article <100tgb5p7aom585@corp.supernews.com>, > manton@nocompusmart.abspam.ca says... > > I am actually facing the design of a power supply for automotive use as > well. > > I came up with the following and would appreciate comments... > > Take 12V DC into a bridge rectifier (~ terminals). Output of rectifier > "+" through poly fuse to power supply circuit. Place a TVS (such as the > above MR2535L) across the "+" & "-" terminals of the rectifier. >As someone else also stated, do not use a bridge, just a single diode in the positive supply line. You should have your circuit referenced to the vehicle ground.> My thinking is if there is an overvoltage the MR2535 alone will eat the > spikes. However, if for some reason there are negative spikes on the > 12V, the TVS alone won't do anything (and other solutions use a second > TVS to shunt negative spikes into the positive terminal).There is no issue with the negative spikes, as the MR2535L is not bidirectional. It looks like a standard zener diode, so since its forward voltage is around a volt, that is what it will clamp your negative spikes to.> > With a rectifier all spikes are positive going & combined with the TVS, > they should be clamped...right?Sure, but only if your rectifier bridge is fast enough, and rated for the high instantaneous currents involved. But the method above is better. Mike Anton
Power supply in automotive environment
Started by ●January 20, 2004
Reply by ●January 23, 20042004-01-23
Reply by ●January 23, 20042004-01-23
Robert Scott wrote:> There is no magic way to make a MOSFET into a low-drop diode. The > bridge rectifiers made out of MOSFETs use additional active circuitry > to drive the gates appropriately. It is not as simple as just > shorting two pins of the MOSFET.Hmmm, makes me go and look it up in my PSU book... "Power Supplies, Switching Regulators, Inverters, and Converters" I was wrong, you don't just short two pins - you sort of cross them over to the opposite side, but the "MOSFET synchronous rectifying bridge" depicted on page 422-423 doesn't require additional active circuitry. Um.. difficult to draw this circuit in ASCII. I'll try. O------+-----------------------+------------+ \ +--------------- | | \ / N/ \P-------------+ \/ / \ | | AC IN /\ -ve O---+ +---O +ve | | / \ \ / | | / \ N\ /P----------+ | / +-------------- | | O-----+------------------------+---------------+ Got that? Where I've put an N or a P, that means there's an N-channel or P-channel power MOSFET there, with the source connected closest to the DC outputs. The inputs that cross over and go to the other side are of course the gates, leaving the drains as the outputs in the diagonal lines opposite the sources, obviously. ABS
Reply by ●January 23, 20042004-01-23
Look at the LM2940. It was designed for vehicular use; it has short-circuit and reverse voltage protection.
Reply by ●January 24, 20042004-01-24
Alaric B Snell wrote:> > Robert Scott wrote: > > There is no magic way to make a MOSFET into a low-drop diode. The > > bridge rectifiers made out of MOSFETs use additional active circuitry > > to drive the gates appropriately. It is not as simple as just > > shorting two pins of the MOSFET. > > Hmmm, makes me go and look it up in my PSU book... > > "Power Supplies, Switching Regulators, Inverters, and Converters" > > I was wrong, you don't just short two pins - you sort of cross them over > to the opposite side, but the "MOSFET synchronous rectifying bridge" > depicted on page 422-423 doesn't require additional active circuitry. > > Um.. difficult to draw this circuit in ASCII. I'll try. > > O-------+-----------------------+------------+ > \ | | > \ +----------------N/ \P-------------+ > \/ / \ | | > AC IN /\ -ve O---+ +---O +ve | | > / \ \ / | | > / +---------------N\ /P----------+ | > / | | > O------+------------------------+---------------+ > > Got that? Where I've put an N or a P, that means there's an N-channel or > P-channel power MOSFET there, with the source connected closest to the > DC outputs. The inputs that cross over and go to the other side are of > course the gates, leaving the drains as the outputs in the diagonal > lines opposite the sources, obviously.This may look good on paper, but if you are worried about large transients, then the gate is likely to not survive. I guess you can find MOSFETs with large gate to drain breakdown voltages, but the ones I have seen are not very high. For example, I don't think you could use this on a 120 VAC circuit, as I don't know of any parts that will take that high a gate voltage. Am I wrong? However, if you use a zener diode and resistor at the gates, you can limit the gate voltage to something that will fully turn on the MOSFET and not blow it out. Or if you are just looking for reverse polarity protection, then you only need one P-channel FET connected like the one on the upper right (and of course the zener to protect the gate).